Micro-organisms such as yeast, bacteria, fungi,
or algae are the single-cell proteins used for most bioconversion
of wastes or other substrates to make food or feed. A crucial
question is: What is the nutritional value for man or animals of
the final product of the bioconversion process? A second
important aspect is the toxicological status of the product. This
is the subject of papers by Scrimshaw and Shacklady appearing
elsewhere in these proceedings, and my presentation is based on
the assumption that the materials are acceptable toxicologically.
I will consider some points that must be taken into account when
evaluating a bioconversion product for animal feeding.

The main reasons for using micro-organisms in
the conversion of agricultural residues are: First, to degrade
that part of the residue that is not available for absorption by
animals or man when the material is fed as such. In most cases
this means that the enzymes secreted in the animal or human
gastro-intestinal tract cannot, or are insufficiently able to,
break down the material into components that can be absorbed.
This pertains to cellulosic, hemicellulosic, and ligno-cellulosic
components. The second purpose is to upgrade the nutritional
quality of the residue by increasing its protein content, or, for
monogastric animals and man, raising its content of essential
amino acids.

Of the four categories of micro-organisms
involved in bioconversion processes (yeasts, bacteria, fungi, and
algae), a considerable amount of information is available about
the nutritional value of yeasts. Species of yeast have been used
for many years as a valuable component of animal feeds, supplying
proteins and certain vitamins. In addition, some of the
large-scale industrial SCP processes developed over the past ten
years use yeasts that utilize hydrocarbons (i.e., paraffins) as
an energy source and carbon and hydrogen for growth and synthesis
of cell constituents. The results of extensive evaluation
programmes show that these yeasts form a highly valuable source
of protein for monogastric animals.

The second category of SCP, the bacteria, have,
for many centuries, contributed to food supplies for man in an
indirect manner: the protein supply of the ruminant is largely
dependent on the bacteria and protozoa abundantly present in the
fore-stomach of the animal, which forms, in principle, a large in
vivo fermentation vessel.

Bacteria will be used in several large units
being constructed for industrial protein production where methane
or methanol will provide the energy. The data available show that
bacterial material produced in this way also forms a highly
valuable protein source.

The last two categories of SCP, the fungi and
algae, have until now not been used to any extent in animal
feeding, and this is why very little is known about the
nutritional value of these products. The scarce data in the
literature show variable results and indicate that, for
monogastric animals, digestibility may be a problem. ILOB
experiments with a fungal product showed reasonable results for
digestibility and growth performance in pigs, but the results in
poultry were unsatisfactory. Because fungi and algae will most
likely be the microorganisms of choice for the small-scale
bioconversion units considered in this work shop, a thorough look
at the nutritional value of the material produced is essential. I
would especially stress the necessity of testing nutritional
value at an early stage of process development in order to be
able to provide some sort of guidance for that development, for
example, the choice of the micro-organism or the relevance of
including a special treatment, if possible.

I shall discuss the procedures that can be
applied when testing the nutritional value of new components
meant for inclusion in animal feeds. Table 1 illustrates three
different possible approaches. The left column shows tests used:
analysis, acceptability, digestibility, and comparative feeding
trials. These four different types of tests form a chronological
sequence of steps in the evaluation procedure. In the columns on
the right the efforts involved in each of the steps are given.
The figures mentioned in these columns are calculated from the
real costs of the experiments under Western European conditions.
For the animal experiments they reflect the type of equipment
needed, number of animals involved, degree of sophistication,
etc. All figures mentioned are calculated as percentages of the
total of "Efforts" column I - that is, the total of the
efforts involved in a complete, elaborate nutritional evaluation
of an SCP developed for production in large industrial operations
in highly industrialized countries. It must be realized that the
number 100 means, according to standards of the wealthy Western
countries, a sum on the order of 400,000 - 500,000 Dutch guilders
or US$200,000 - 250,000, and only involves the nutritional
evaluation per se, not special toxicological determinations, or
time-consuming, expensive multi-generation studies.

"Efforts I" is the most sophisticated
evaluation procedure. An evaluation always starts with an
elaborate analysis step, including determination of the major
components: protein, fat, ash, carbohydrate, but preferably also
amino acids, macro elements, and the more important minor
elements. Because of the very modest costs involved, it is
advisable to perform as many of the relevant analyses as
possible, as a complete analytical profile can provide much early
information about the potential nutritional value of the product
to be tested.

The second step, acceptability trials, includes
small, short-run tests with chicks and pigs. Sheep are not
included in column I because, in highly developed countries, SCP
is usually too expensive a source of protein for ruminants. These
acceptability studies determine whether the inclusion of a
moderate and a high dose of the test-product in the diet affects
feed intake, faeces consistency, etc., as indicators of
digestibility and general state of health. Weight gain is also
measured as a first, very rough estimate of nutritional value.

The third step in the evaluation programme,
determination of digestibility, is a very important one because
the result is, to a great extent, a determination of the
nutritional value of the product under test. In column 1,
complete digestibility trials with chicks and pigs are
anticipated. In these trials energy value (metabolizable energy
for chicks, digestible energy for pigs) is also determined.

After the results of these first three phases
of the programme have become available, a reasonably reliable
prediction of nutritional value can be given, provided the test
product does not contain specific negative factors not discovered
in the acceptability and digestibility trials. The prediction is
verified in the last, fourth phase: the comparative feeding
experiments. In addition to chicks and pigs, these trials also
include an experiment with laying hens. In these experiments, the
test product replaces part or all of the usual high-protein
components in the diet in order to see how it affects weight
gain, egg production, feed conversion efficiency, and product
quality.

I should like to emphasize that phase 2
(acceptability) and especially phase 4 (comparative feeding)
allow inclusion of toxicological determinations, because the
target animals are consuming moderate to high levels of the test
product over a prolonged period. In some programmes evaluating
commercial products (ILOB work on B.P. yeast, Imperial Chemical
Industries, Ltd. [ICI] in the evaluation of the ICI bacterial
product), multi generation feeding studies with laying hens and
breeding pigs were also included in order to assess the
reproductive capacity of these farm animals, and to detect any
long-term effects.

TABLE 1. Three Approaches for Determination of
Nutritional Value of New Components for Animal Feed

Efforts (%)

Type of test

I

II*

III*

a

b

1. Analysis

0.5

0.5

0.5

0.5

chicks

1.5

1.5

1.5

1.5

2
Acceptability

pigs/sheep

2

2

2

2

chicks

10

4

4

4

3.
Digestibility

pigs/sheep

14

6

-

6

chicks

14

3

3

-

4.
comparative

layers

27

5

-

-

pigs/sheep

31

8

-

5

100%

30%

11%

19%

Percentage of the total cost of scheme I
attributable to individual components. 100% represents 400,000
500,000 guilders or us$200,000 - 250,000 at 1978 prices.

Moderately simplified and greatly reduced
evaluation schemes are given in columns 11 and l l l
respectively, although these are based on the same principles and
follow the same sequence of trials as in the sophisticated,
elaborate evaluation scheme of column 1. In all stages of the
animal experiments, pigs and sheep are given as an alternative,
depending on local conditions. It is assumed that the efforts
involved in experiments with pigs and sheep are approximately
equal. In both reduced schemes, phases 1 (analysis) and 2
(acceptability trials) are maintained to the same extent because
they are relatively simple and cheap and yield much useful
information. In phase 3, determination of digestibility, the
study is considerably simplified because the test is carried out
with only one level of the test product in the diet in one test
period instead of the two test periods used in column 1. The
result of this simplification is that the figures for
digestibility become less reliable, but when the study is
carefully done, a good measurement of nutritional value can still
be obtained. It is possible that an even simpler test for
digestibility could be used by an in vitro method, but it remains
to be determined whether existing in vitro methods give reliable
results with the kinds of SCP products under discussion in this
paper.

TABLE 2. Effects on Growth and Mortality in
Chicks Fed Fishmeal and Yeast Diets with and without Vitamin E
and Arginine

Diets

Weight
gain

0 - 5 weeks (%)

Mortality
1%)

30% Fishmeal

100

2

30% Fishmeal
+ vitamin E

95

5

30% Fishmeal
+ vitamin E + arginine

122

6

30% Yeast

50

70

30% Yeast +
vitamin E

58

47

30% Yeast +
vitamin E + arginine

68

63

The experiments mentioned in phase 4,
comparative feeding trials, are also simpler in terms of number
of animals per trial and duration of the trials. In the
moderately reduced scheme 11 a six-month experiment with laying
hens fed one level of test product is still included; in scheme
III it is omitted, and it is assumed that the results in chicks
will give a sufficient predictability of the effects on laying
hens.

The total effort involved in scheme 11 is only
30 per cent of that in scheme I and it is reduced to 11 and 19
per cent, respectively, in schemes III a and b. The difference
between schemes III a and b is as follows. In scheme III a it is
assumed that chicks will provide satisfactory information on
digestibility. A further testing of the product in pigs or sheep
can be omitted because it is very likely that a product showing
satisfactory results in poultry will have an equally good, or
even better, nutritional value for pigs or sheep. In scheme III a
the chick test of digestibility is followed only by a five week
comparative (chick experiment) feeding study with chicks.

When the chick digestibility test does not show
promising results, we use scheme III b and continue with a check
of digestibility in pigs or sheep and, if relevant, follow this
with a comparative feeding trial in pigs or sheep.

The schemes shown in the table, of course, do
not mean that these are strictly defined programmes that must be
used, but they can be considered as guidelines that can be
followed, depending on local conditions and circumstances. As has
been noted before, the scheme indicated in column I is a
full-scale, elaborate nutritional evaluation programme of an SCP
from a large industrial operation. The moderately simplified
scheme 11 can be used to test a product from a small industrial
plant, for example, a regional co-operative.

TABLE 3. Effects on Growth and Mortality in
Chicks Fed Fishmeal and Yeast Diets with and without Selenium

Diets

Weight
gain

0 - 3 weeks (%)

Mortality

(%)

30% Fishmeal

100

7

30% Fishmeal
+ 0.2 ppm selenium

101

3

30 % Yeast

70

53

30% Yeast +
0.2 ppm selenium

80

2

The markedly trimmed schemes III a and b may be
used in the evaluation of material from a village or
multi-village unit.

In conclusion, I should like to present one
example to illustrate that, when evaluating a new product, care
must be taken not to confuse nutritional defects with toxicity.
During the nutritional evaluation of yeast grown on e-paraffins
or gas oil, an experiment was carried out in which chicks were
fed semi-synthetic diets with fishmeal or Yeast, respectively, as
the sole source of protein. After four weeks, about 50 per cent
of the yeast-fed chicks had died, whereas only one of the
fishmeal-fed chicks had dies. Because at that stage of the yeast
evaluation we already knew from the results of rat and other
chick experiments that true toxicity was a very unlikely
explanation for the phenomenon observed, we undertook a closer
look at the nutritional characteristics of the yeast, as
summarized in Tables 2-4.

In the experiment just described, many birds
showed symptoms pointing to vitamin E and/or arginine deficiency,
so the effect of the addition of vitamin E and arginine was
studied. In the fishmeal-fed controls, mortality was always low
but adding arginine improved weight gain distinctly. In the
unsupplemented yeast group, mortality was 70 per cent and weight
gain 50 per cent of that in fishmeal-fed controls. Addition of
vitamin E improved weight gain only slightly, but mortality was
considerably lower. Addition of both vitamin E and arginine
effected a further improvement in growth, but mortality rose once
again.

The problem was for the most part solved in the
following experiment by the addition of 0.2 ppm selenium. Vitamin
E and arginine were added to all rations. Whereas selenium had
hardly any effect on chicks consuming the fishmeal diet, it
dramatically reduced mortality of chicks on the yeast diet. In
addition, weight gain was improved by 10 per cent, although still
lagged 20 per cent behind the fishmeal-fed controls (Table 3).
This fairly considerable growth depression proved to be caused by
the extremely fine, sandy structure of the yeast, which
apparently limited feed intake when the diet was fed as meal.

TABLE 4. Effects on Growth and Mortality in
Chicks Fed Fishmeal and Yeast Diets in the Form of Meal or
Pellets

Diets

Weight
gain

0 - 3 weeks (%)

Mortality

(%)

Meal

30% Fishmeal

100

5

30% Yeast

74

0

Pellets

30% Fishmeal

100

2

30% Yeast

98

2

The problem could be completely overcome by
pelleting the final feed, as shown in Table 4, which gives the
results of an experiment in which meal and pelleted diets were
compared. Selenium, arginine, and vitamin E were added to both
meal and pelleted diets.

This example was chosen because it gives such a
clear illustration of the dramatic effects of a combination of
nutritional imbalances, in this case, selenium, vitamin E, and
arginine, and effects of the structure of the feed, that might
easily have been considered to be symptoms of real toxicity. In
this example, the effects were somewhat exaggerated because of
the use of semi-synthetic diets in which the test product was the
only protein source. In the usual, less

comprehensive evaluation schemes, where the
experimental conditions are less extreme - i.e., more realistic
test diets and mixtures of proteins - the effects would be less
marked, but nonetheless important.

The moral of the story is that, in the process
of evaluating a new product, it is essential for the nutritionist
and the toxicologist to co-operate as closely as possible. A
reliable toxicological evaluation of a product can only be done
when the main nutritional characteristics of that product are
known; on the other hand, the nutritionist must know all the
available toxicological data before he can complete the
nutritional evaluation.

Finally, I should like to emphasize once more
that very little is known about the nutritional and toxicological
properties of fungi and algae, the micro-organisms most relevant
for bioconversion processing of agricultural residues. In my
opinion, we should urge research in the field as strongly as
possible. As important research targets regarding nutritional
value, especially for monogastric animals, I would include:
Determination of the nutritional value of different relevant
species of fungi and algae grown on different substrates and
under different conditions; investigation of the influence of the
cell wall on this nutritional value and, in connection with this,
the effect of special processing methods such as drying, milling,
and pelleting.